linux-kernel - Re: [PATCH v3] libbpf: workaround -Wmaybe-uninitialized false positive

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Message-ID: <61cf5568-7a01-4231-8189-006bde4ec0ad@oracle.com>
Date: Mon, 12 Aug 2024 14:56:32 +0100
From: Alan Maguire <alan.maguire@...cle.com>
To: Sam James <sam@...too.org>, Andrii Nakryiko <andrii@...nel.org>,
        Eduard Zingerman <eddyz87@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>
Cc: "Jose E . Marchesi" <jose.marchesi@...cle.com>,
        Andrew Pinski <quic_apinski@...cinc.com>,
        Kacper Słomiński <kacper.slominski72@...il.com>,
        Arsen Arsenović <arsen@...too.org>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] libbpf: workaround -Wmaybe-uninitialized false
 positive

On 12/08/2024 11:37, Sam James wrote:
> In `elf_close`, we get this with GCC 15 -O3 (at least):
> ```
> In function ‘elf_close’,
>     inlined from ‘elf_close’ at elf.c:53:6,
>     inlined from ‘elf_find_func_offset_from_file’ at elf.c:384:2:
> elf.c:57:9: warning: ‘elf_fd.elf’ may be used uninitialized [-Wmaybe-uninitialized]
>    57 |         elf_end(elf_fd->elf);
>       |         ^~~~~~~~~~~~~~~~~~~~
> elf.c: In function ‘elf_find_func_offset_from_file’:
> elf.c:377:23: note: ‘elf_fd.elf’ was declared here
>   377 |         struct elf_fd elf_fd;
>       |                       ^~~~~~
> In function ‘elf_close’,
>     inlined from ‘elf_close’ at elf.c:53:6,
>     inlined from ‘elf_find_func_offset_from_file’ at elf.c:384:2:
> elf.c:58:9: warning: ‘elf_fd.fd’ may be used uninitialized [-Wmaybe-uninitialized]
>    58 |         close(elf_fd->fd);
>       |         ^~~~~~~~~~~~~~~~~
> elf.c: In function ‘elf_find_func_offset_from_file’:
> elf.c:377:23: note: ‘elf_fd.fd’ was declared here
>   377 |         struct elf_fd elf_fd;
>       |                       ^~~~~~
> ```
> 
> In reality, our use is fine, it's just that GCC doesn't model errno
> here (see linked GCC bug). Suppress -Wmaybe-uninitialized accordingly
> by initializing elf_fd.elf to -1.
> 
> I've done this in two other functions as well given it could easily
> occur there too (same access/use pattern).
>

hmm, looking at this again - given that there are multiple consumers -
I suppose another option would perhaps be to

- have elf_open() to init int fd = -1, Elf *elf = NULL.
- have error paths in elf_open() "goto out"; at out: we set elf_fd->fd,
elf_fd->elf to fd, elf
- have elf_close() exit it elf_fd < 0 (since 0 is a valid fd), as it
will for the error cases

Might all be bit excessive, and might not even fix the false positive
issue here, so

> Link: https://gcc.gnu.org/PR114952
> Signed-off-by: Sam James <sam@...too.org>

Reviewed-by: Alan Maguire <alan.maguire@...cle.com>

> ---
> v3: Initialize to -1 instead of using a pragma.
> 
> Range-diff against v2:
> 1:  8f5c3b173e4cb < -:  ------------- libbpf: workaround -Wmaybe-uninitialized false positive
> -:  ------------- > 1:  12cec1262be71 libbpf: workaround -Wmaybe-uninitialized false positive
> 
>  tools/lib/bpf/elf.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/lib/bpf/elf.c b/tools/lib/bpf/elf.c
> index c92e02394159e..00ea3f867bbc8 100644
> --- a/tools/lib/bpf/elf.c
> +++ b/tools/lib/bpf/elf.c
> @@ -374,7 +374,7 @@ long elf_find_func_offset(Elf *elf, const char *binary_path, const char *name)
>   */
>  long elf_find_func_offset_from_file(const char *binary_path, const char *name)
>  {
> -	struct elf_fd elf_fd;
> +	struct elf_fd elf_fd = { .fd = -1 };
>  	long ret = -ENOENT;
>  
>  	ret = elf_open(binary_path, &elf_fd);
> @@ -412,7 +412,7 @@ int elf_resolve_syms_offsets(const char *binary_path, int cnt,
>  	int err = 0, i, cnt_done = 0;
>  	unsigned long *offsets;
>  	struct symbol *symbols;
> -	struct elf_fd elf_fd;
> +	struct elf_fd elf_fd = { .fd = -1 };
>  
>  	err = elf_open(binary_path, &elf_fd);
>  	if (err)
> @@ -507,7 +507,7 @@ int elf_resolve_pattern_offsets(const char *binary_path, const char *pattern,
>  	int sh_types[2] = { SHT_SYMTAB, SHT_DYNSYM };
>  	unsigned long *offsets = NULL;
>  	size_t cap = 0, cnt = 0;
> -	struct elf_fd elf_fd;
> +	struct elf_fd elf_fd = { .fd = -1 };
>  	int err = 0, i;
>  
>  	err = elf_open(binary_path, &elf_fd);